FYI, if you need to travel after your greencard expires but before you get your new one (or if the greencard will expire while you're in the middle of a trip), get your passport stamped before you leave. I believe you can get this done at an Infopass interview. The receipt letter you review after submitting the I-90 isn't enough to get back into the US (i.e., you will probably be let in, but only after you go through secondary review at the border, get hassled by some incredibly rude civil servant and miss any connecting flights).